It'll pay off in the end, but be prepared for a number of lj_maintenance posts this week.
As far as end-user effect:
-- you may see your journal's in read-only mode for a minute or two. that probably just means it's in the process of moving between database clusters.
-- userpics may be flaky for a couple days while we shuffle that around. hopefully we're across all the major hurdles, though.
We'll keep you updated.